1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and in particular, to a method for fabricating an LCD having photoalignment layers on the inner surfaces of two substrates that bond those substrates together using light radiation.
2. Discussion of the Related Art
Because of their size, weight, low power, and high image quality, LCDs are replacing cathode ray tubes (CRT) in many applications. In general, an LCD comprises a first substrate, a second substrate, and a liquid crystal between those substrates. There are many types of liquid crystals, including nematic, smectic, and cholesteric liquid crystals.
The liquid crystal types are classified according to their molecular array structure. The nematic liquid crystal has an irregular molecular array, but the molecules of the nematic phase have nearly parallel arrangement. The smectic liquid has a higher state of order than the nematic liquid crystal. That is, the smectic liquid crystal also has a layer structure, and molecules are randomly arrayed in each layer. The cholesteric liquid crystal has a rotational characteristic, like the smectic liquid crystal, such that the axes of the molecules are rotationally twisted.
A smectic liquid crystal has a switching speed several hundred times faster than a nematic liquid crystal. This speed advantage reduces screen image vagueness, making the smectic liquid crystal more suitable for large screen displays. Also, a smectic liquid crystal has a dual stability (or memory) that produces good image quality without flicker.
However, a smectic liquid crystal has drawbacks. They are vulnerable to external shocks and are highly degraded once their molecular orientations are broken. Such drawbacks are minimized by replacing the conventional ball spacers with shock resistant patterned spacers or adhesive spacers.
Methods for fabricating a smectic LCD will now be described. FIGS. 1A to 1D illustrate a first conventional process of fabricating a smectic LCD, while FIGS. 2A to 2D illustrate a second conventional process.
As shown in FIG. 1A, pixels are defined on a first substrate 11 by crossing gate lines and data lines. Thin film transistors are formed at the crossings. Pixel electrodes 13 that electrically connect to the thin film transistors are then formed. Thereafter, patterned spacers 15 are formed by photolithography. In practice, the patterned spacers 15 should be formed on the gate and data lines between the pixel electrodes.
The second substrate 12 includes a black matrix that prevents light leakage and that enhances a contrast ratio; red, green and blue color filters to produce colors; and a common electrode 14 that faces the pixel electrodes 13.
Referring now to FIG. 1B, the first substrate 11 and the second substrate 12 are coated with orientation films 16a and 16b. Then, as shown in FIG. 1C, the substrates are disposed facing each other. Those facing substrates are thermally pressed together to induce chemical bonding between the orientation films 16a and 16b and to bond the two substrates together. The shock resistance of the LCD is enhanced by the orientation films. In other words, external shocks are absorbed by the orientation films on the patterned spacers 15.
Eventually, the smectic LCD is completed by interposing a smectic liquid crystal 19 between the first substrate 11 and the second substrate 12 as shown in FIG. 1D.
Here, the liquid crystal orients in a predetermined direction due to the orientation films 16a and 16b, for which anisotropy have been provided. Depending on the application, a nematic or cholesteric liquid crystal can be used in place of the smectic liquid crystal.
According to another conventional technology, the smectic LCD can be fabricated as follows. As shown in FIG. 2A, crossing gate lines and data lines for transferring scan signals and data signals are arrayed on a first substrate 21. Thin film transistors are formed at the crossings. Pixel electrodes 23 that electrically connect to the thin film transistors are then formed. A first oriented film 26a is coated over the first substrate, include the thin film transistors and pixel electrodes 23.
A black matrix that corresponds to the gate and data lines, and to the thin film transistors, is formed on a second substrate 22. Color filters are formed between openings in the black matrix. A common electrode 24 is then formed over the second substrate 21, including over the black matrix and color filters. The common electrode 24 will face the pixel electrodes 23. A second oriented film 26b is then coated over the common electrode 24.
Then, the first and the second oriented films 26a and 26b are calcinated, and the surfaces thereof are rubbed by means of a rubbing roll surrounded by a cloth of a particular type. That rubbing induces an alignment direction to the oriented films 26a and 26b. 
Referring now to FIG. 2B, ball spacers 27 are then dispersed over the first oriented film 26a of the first substrate 21. Additionally, adhesive spacers 28 are evenly dispersed over the first substrate 21.
The first substrate 11 and the second substrate 12 are then disposed facing each other as shown in FIG. 2C. The ball spacers 27, which are smaller than the adhesive spacers, are used to space the two substrates at a predetermined distance. The adhesive spacers bond the two substrates together and absorb shocks.
The adhesive spacers 28 are calcinated and completely bonded by thermally pressing the two substrates together as shown in FIG. 2D. An LCD is completed by interposing a liquid crystal 29 between the two substrates.
The liquid crystal 29 stably orients at a predetermined direction due to the anisotropy of the oriented films. Any one of the nematic, smectic or cholesteric liquid crystal may be used for the liquid crystal 29.
However, the conventional methods of fabricating a smectic LCD as described above have problems. The conventional LCD necessitates separate processes for calcinating the oriented films and calcinating the first and the second substrates for bonding. Furthermore, treatment of the oriented films by mechanical rubbing causes contamination due to dust, as well as damage of the thin film transistors due to static electricity. Both reduce the reliability of the LCD. Moreover, it is difficult to evenly orient the films by rubbing.